`null` and `undefined` events are handled differently

[hyunbinseo]

Describe the bug

All other attributes are included unless their value is nullish (null or undefined).

https://svelte.dev/docs/svelte/basic-markup#Element-attributes

Because events are just attributes, the same rules as for attributes apply:

https://svelte.dev/docs/svelte/basic-markup#Events

However, ondblclick={null} is treated differently in the JS output:

<button name={undefined} value={null} onclick={undefined} ondblclick={null}></button>

$.set_attribute(button, 'name', undefined);
$.set_value(button, null);
button.__click = undefined;

button.__dblclick = function (...$$args) {
  (null)?.apply(this, $$args);
};

Shouldn't this be button.__dblclick = null; instead?

Reproduction

https://svelte.dev/playground/d6dd8c1f6355475fbe357d501b60137a?version=5.38.6

Logs

System Info

-

Severity

annoyance

[Ocean-OS]

undefined is a valid variable name, so you could do something like this where undefined is an event handler and it would all be valid JS. However, null is a literal, like numbers or strings. You cannot make a variable named null.
As such, undefined is treated differently from null at this level. (Not to mention that in the end, if undefined is indeed not defined, the runtime behavior is the same anyways)

[hyunbinseo]

I thought undefined and null were not included in the compiled JS in the first place.

All other attributes are included unless their value is nullish (null or undefined).

That does not seem to be the case, and there seem to be valid reasons.

Thanks for the explanation.

[7nik]

on* attributes are listeners and treated separately. And sane people won't write onclick={undefined} ondblclick={null} so it doesn't make sense to optimize this.